git-gui: Added support for pulling from default branch of a remote.

git-gui: Added support for pulling from default branch of a remote.

We now create one menu entry per remote listing the first Pull: or fetch
entry associated with that remote as the branch to pull into the current
branch.

This is actually quite incorrect as we should be using the default
remote branch name listed in branch.<name>.merge for a new-style remote
described in the config file.  But its a good default to get started with.

Signed-off-by: Shawn O. Pearce <redacted>